Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item MULTISCALE RADIATION-MHD SIMULATIONS OF COMPACT STAR CLUSTERS(2023) He, ChongChong; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Star formation is a crucial process that lies at the center of many important topics in astrophysics: the nature of the first sources of radiation, the formation and evolution of galaxies, the synthesis of elements, and the formation of planets and life. Recent advances in computing technology have brought about unprecedented opportunities to deepen our understanding of this complex process. In this dissertation, I investigate the physics of star formation in galaxies and its role in shaping the galaxies and the Universe through numerical simulations.My exploration of star formation begins with a large set of simulations of star cluster formation from isolated turbulent Giant Molecular Clouds (GMCs) with stellar feedback using \ramses{}, a state-of-the-art radiation-magneto-hydrodynamic (radiation-MHD) code. While resolving the formation of individual stars, I have pushed the parameters (mass and density) of the simulated GMCs well beyond the limit explored in the literature. I establish physically motivated scaling relationships for the timescale and efficiency of star formation regulated by photoionization feedback. I show that this type of stellar feedback is efficient at dispersing dense molecular clouds before the onset of supernova explosions. I show that star formation in GMCs can be understood as a purely stochastic process, where instantaneous star formation follows a universal mass probability distribution, providing a definitive answer to the open question of the chronological order of low- and high-mass star formation. In a companion project, I publish the first study of the escape of ionizing photons from resolved stars in molecular clouds into the intercloud gas. I conclude that the sources of photons responsible for the epoch of reionization, one of the most important yet poorly understood stages in cosmic evolution, must have been very compact star clusters, or globular cluster progenitors, forming in dense environments different from today's galaxies. In follow-up work, I use a novel zoom-in adaptive-mesh-refinement method to simulate the formation and fragmentation of prestellar cores and resolve from GMC scales to circumstellar disk scales, achieving an unprecedented dynamic range of 18 orders of magnitude in volume in a set of radiation-MHD simulations. I show that massive stars form from the filamentary collapse of dense cores and grow to several times the core mass due to accretion from larger scales via circumstellar disks. This suggests a competitive accretion scenario of high-mass star formation, a problem that is not well understood. We find that large Keplerian disks can form in magnetically critical cores, suggesting that magnetic braking fails to prevent the formation of rotationally-supported disks, even in cores with mass-to-flux ratios close to critical. This is because the magnetic field is extremely turbulent and incoherent, reducing the effect of magnetic braking by roughly one order of magnitude compared to the perfectly aligned and coherent case, which proposes a solution to the ``magnetic braking catastrophe.''Item MOLECULAR SPECTROSCOPY OF STAR FORMING REGIONS: COOL AND HOT, CLOSE AND FAR(2023) Li, Jialu; Harris, Andrew; Tielens, Alexander; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Star formation processes originating from dense molecular clouds leave us a molecular universe. How molecules probe the physical conditions at different star-forming stages and how the physical environments control the formation of the chemical inventory becomes a key question to pursue. In the past, the understanding of this problem is impeded by instrumentallimitations. With instruments advanced in sensitivity and spatial/spectral resolution, this thesis investigates the molecular environment of different star-forming regions. Half of this thesis (Chapter 2 and Appendix A) focuses on mapping cold dense molecular gas in an external galaxy, IC 342, at 3 Mpc. The distribution of molecular gas was efficiently mapped with a set of density-sensitive tracers with Argus. Argus is the first array receiver functioning at 3 mm on the 100 m Green Bank Telescope (GBT) and provides a resolution of 6′′–10′′. As this study was conducted in the early era of Argus’ deployment, valuable information on the instrument’s behavior is learned. The resolved molecular maps characterize the fundamental physical properties of the clouds including the volume density and the excitation conditions. Comparisons with results from radiative transfer modeling with RADEX help to decrypt this information. The high spatial resolution of Argus also provides an opportunity in inspecting a scale-scatter breakdown of the gas density-star formation correlation in nearby galaxies and in investigating the influence of a finer spatial resolution on the correlation. The other half of the thesis (Chapters 3 and 4) studies the hot core, an embedded phase during massive star formation, of a proto-binary system W3 IRS 5 at 2.2 kpc. Rovibrational transitions of gaseous H2O, CO, and isotopologues of CO were detected with mid-IR absorption spectroscopy. The high spectral resolution (R ∼50,000–80,000) not only separates each transition individually but also decomposes different kinematic components residing in the system with a velocity resolution of a few km/s . Physical substructures such as the foreground cloud, high-speed “bullet”, and hot clumps in the disk surface are identified. Characterization of the physical substructures is conducted via the rotation diagram analysis and curve-of-growth analyses. The curve-of-growth analyses, under either a foreground slab model or a disk model, take account of the optical depth effects and correct the derived column densities by up to two orders of magnitude. The disk model specifically suggests a disk scenario with vertically-decreasing temperature from mid-plane, which is intrinsically different from externally illuminated disks in the low-mass protostellar systems that have hot surfaces. Connections between physicalsubstructures and chemical substructures were also established. Investigations on chemical abundances along the line of sight reveal the elemental carbon and oxygen depletion problem.Item Gas and Star Formation at the Peak of Cosmic Star Forming Activity(2021) Lenkic, Laura; Bolatto, Alberto; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gas and star formation in galaxies are intimately linked to one another. Molecular hydrogen gas is the material out of which stars form, while the process of forming stars, in turn, depletes the reservoirs of gas in galaxies and builds up their stellar mass. Observations of star formation in galaxies over time indicate that they must form stars for timescales longer than would be expected from their gas content and star formation rates, indicating that processes that replenish the star forming fuel must be present. The focus of this thesis is on two components of this qualitative picture: the molecular hydrogen gas content of galaxies over time, and the link between gas and star formation in galaxies resembling those observed at the epoch of most active star formation. First, I present a systematic search for serendipitous carbon monoxide emitting sources in the second Plateau de Bure High-z Blue-Sequence Survey (PHIBSS2). These observations presented an opportunity to quantify the mass density of molecular gas in galaxies as a function of time, and to link this to the star formation history of the Universe. I use a match-filter technique to systematically detect 67 serendipitous sources, after which I characterize their properties, creating a catalog of their redshifts, line widths, fluxes, estimations of the detection reliability, and completeness of the detection algorithm. I find that these serendipitous sources are unrelated to the primary sources that were targeted by PHIBSS2, and use the catalog to construct luminosity functions spanning a redshift range from $\sim 0.3-5$. From these luminosity functions, I place constraints on the molecular hydrogen content in galaxies over cosmic time. My work presents one of the first attempts to use existing observations for this measurement and yields results that are consistent with previous studies, while demonstrating the scientific power of large, targeted surveys. Next, I study a sample of rare, nearby galaxies that are most similar to those we observe at the peak of cosmic star forming activity that occured $\sim 10$ billion years ago. These galaxies are drawn from the DYnamics of Newly Assembled Massive Objects (DYNAMO) survey, and their proximity to us allows for very detailed studies of their massive star forming clumps. I use observations from the Hubble Space Telescope (HST) to measure colors that are sensitive to stellar population age and extinction. From these measurements, I find that clumps in DYNAMO galaxies have colors that are most consistent with very young centers and outskirts that appear systematically older, by as much as 150~Myr in some cases. I attribute this age difference to the presence of ongoing star formation in the centers of clumps that maintains the population of massive, short-lived stars and gives rise to colors consistent with young ages. Furthermore, I find that within the disks of their host galaxies, younger clumps are preferentially located far from galaxy centers, while older clumps are preferentially located closer to the centers. These results are consistent with hydrodynamic simulations of high-redshift clumpy galaxies that predict clumps form in the outskirts of galaxies via a violent disk instability, and as they age, migrate to the centers of galaxies where they merge and contribute to the growth of galactic bulges. Building on this study, I combine observations of DYNAMO galaxies from the HST and the Atacama Large Millimeter/sub-millimeter Array (ALMA) to trace molecular hydrogen gas and star formation. I link these observations to measurements of the molecular gas velocity dispersions to test theories of star formation. I find that compared to local samples of ``normal'' star forming galaxies, DYNAMO systems have consistently high velocity dispersions, molecular gas surface densities, and star formation rate surface densities. Indeed, throughout their disks, DYNAMO galaxies are comparable to the centers of local star forming galaxies. Stellar bar driven gas flows into the centers of galaxies in these local samples may give rise to the high observed velocity dispersions, and gas and star formation rate surface densities. For DYNAMO galaxies, the widespread elevated values of these parameters may be driven by galactic-scale gas inflows, which is predicted by theories. Finally, current theories of star formation, such as the feedback regulated model, assume that turbulence dissipates on timescales proportional to the angular velocity of a galaxy (eddy or crossing time). Yet, I find such models have difficulty reproducing the DYNAMO measurements, and thus conclude that the turbulent dissipation timescale in DYNAMO galaxies must scale with galactocentric radius.Item The Star-Forming Properties of an Ultra-Hard X-ray Selected Sample of AGN(2016) Shimizu, Thomas Taro; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis provides a comprehensive examination of star formation in the host galaxies of active galactic nuclei or AGN. AGN are bright, central regions of galaxies that are powered through accretion onto a supermassive black hole (SMBH). Through accretion and the loss of gravitational potential energy, AGN emit powerful radiation over all wavelengths of the electromagnetic spectrum. This radiation can influence the AGN's host galaxy through what is known as AGN ``feedback'' and is thought to suppress star formation as well as stop accretion onto the SMBH leading to a co-evolution between the SMBH and its host galaxy. Theoretical models have long invoked AGN feedback to be able reproduce the galaxy population we see today but observations have been unclear as to whether AGN actually have an effect on star formation. To address this question, we selected a large sample of local ($z < 0.05$) AGN based on their detection at ultra-hard X-ray energies (14--195 keV) with the \textit{Swift} Burst Alert Telescope (BAT). Ultra-hard X-ray selection frees our sample from selection effects and biases due to obscuration and host galaxy contamination that can hinder other AGN samples. With these 313 BAT AGN we conducted a far-infrared survey using the \herschel \textit{Space Observatory}. We use the far-infrared imaging to probe the cold dust that traces recent star formation in the galaxy and construct spectral energy distributions (SEDs) from 12--500 \micron. We decompose the SEDs to remove the AGN contribution and measure infrared luminosity which provides us with robust estimates of the star formation rate (SFR). Through a comparison with a stellar-mass matched non-AGN sample, we find that AGN host galaxies have larger dust masses, dust temperatures, and SFRs, confirming the results of previous studies that showed the optical colors of the BAT AGN are bluer than non-AGN. We find that the AGN luminosity as probed by the 14--195 keV luminosity is not related to the SFR of the host galaxy suggesting global, large scale star formation on an individual basis is not affected by the AGN. However, after a thorough analysis comparing our AGN to star-forming main sequence, a tight relationship between the SFR and stellar mass of a galaxy, we discover that our AGN as a whole show systematically lower specific SFRs (SFR/stellar mass). We confirm that AGN host galaxies, as a population, are transitioning between the star-forming and quiescent populations. This result supports the theory that AGN feedback has suppressed star formation, but we also consider other models that could reproduce our observations. Finally we conclude with a summary of this thesis and describe several ongoing and future projects that will push forward the exciting field of AGN research.Item Dust and Molecular Gas in the Winds of Nearby Galaxies(2015) McCormick, Alexander; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Galactic winds provide a fundamental mechanism for galaxy evolution. The outflow of material in winds remains the most likely culprit responsible for a host of galaxy observations, plus mounting evidence for galactic winds at times in the past points to their importance in understanding the history of the universe. Therefore, detailed observations of galactic winds are critical to fleshing out the narrative of galaxy evolution. In particular, the dust and molecular gas of a galaxy's interstellar medium (ISM) play crucial roles in the absorption, scattering, and reemission of starlight, the heating of the ISM, and provide critical materials for star formation. We present results from archival {\em Spitzer Space Telescope} data and exceptionally deep {\em Herschel Space Observatory} data of the dust and molecular gas found in and around 20 nearby galaxies known to host galactic-scale winds. Selecting nearby galaxies has allowed us the resolution and sensitivity to differentiate dust and molecular gas outside the galaxies and observe their typically faint emission. These are the most detailed surveys currently available of the faint dust and molecular gas components in galactic winds, and we have utilized them to address the following questions: i) What are the location and morphology of dust and molecular gas, and how do these components compare with better known neutral and ionized gas features? ii) How much do dust and molecular gas contribute to the mass and energy of galactic winds? iii) Do the properties of the dust and molecular gas correlate with the properties of the wind-hosting galaxy? {\em Spitzer} archival data has revealed kiloparsec-scale polycyclic aromatic hydrocarbon (PAH) structures in the extraplanar regions of nearly all the wind-hosting galaxies we investigated. We found a nearly linear correlation between the extraplanar PAH emission and the total infrared flux, a proxy for star formation. Our results also suggest a correlation between the height of extraplanar PAH emission and star formation rate surface density, supporting the idea of a surface density threshold on the energy or momentum injection rate for producing detectable extraplanar wind material. New, very deep {\em Herschel} data of six nearby dwarf galaxies with known winds show circumgalactic cold dust features on galactic scales, often well beyond the stellar component. Comparisons of these features with ancillary data show an imperfect spatial correlation with the ionized gas and warm dust wind components. We found $\sim$10-20\% of the total dust mass in these known wind galaxies resides outside their stellar disks, and $\sim$70\% in one case. Our data also hint at metallicity depletion via cold dust ejection and possible correlations of dust and other host galaxy properties, though these tantalizing implications are not statistically significant given the small number of objects in the sample and the uncertainties in the measurements.Item An efficient method for radiation hydrodynamics in models of feedback-regulated star formation(2013) Skinner, Michael Aaron Reed; Ostriker, Eve C; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We describe a module for the Athena code that solves the gray equations of radiation hydrodynamics (RHD), based on the first two moments of the radiative transfer equation. We combine explicit Godunov methods to advance the gas and radiation variables including non-stiff source terms with a local implicit method to integrate stiff source terms. We adopt the M1 closure relation, including leading source terms to $mathcal{O}(betatau)$ and employ the reduced speed of light approximation (RSLA) with subcycling of the radiation variables to reduce computational costs. We consider self-gravitating fragmentation and evolution of turbulent gaseous clouds, modeling the propagation and interaction of radiation from embedded star clusters that form with the surrounding gas. To model the luminosity sources, we use the star particle algorithm of Gong & Ostriker (2013) based on the particle mesh method combined with an efficient open boundary condition Poisson solver for the self-gravitational potential. Our code is dimensionally unsplit in one, two, and three space dimensions and is parallelized using MPI. The streaming and diffusion limits are well-described by the M1 closure model, and our implementation shows excellent behavior for a problem with a concentrated radiation source containing both regimes simultaneously. Our operator-split method is ideally suited for problems with a slowly-varying radiation field and dynamical gas flows in which the effect of the RSLA is minimal. We present an analysis of the dispersion relation of RHD linear waves highlighting the conditions of applicability for the RSLA. To demonstrate the accuracy of our method, we utilize a suite of radiation and RHD tests covering a broad range of regimes, including RHD waves, shocks, and equilibria, showing second-order convergence in most cases, and a test to demonstrate the accuracy of particle orbits obtained using our method. Applying our method to the study of feedback-regulated star formation in models of giant molecular clouds, we conclude that the radiation force on dust from reprocessed radiation is an efficient mechanism for cloud disruption, which may be particularly important in super star clusters with deep gravitational potential wells.Item Dense Core Formation and Collapse in Giant Molecular Clouds(2013) Gong, Hao; Ostriker, Eve C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we present a unified model for dense core formation and collapse within post-shock dense layers inside giant molecular clouds. Supersonic converging flows collide to compress low density gas to high density clumps, inside which gravitational collapse can happen. We consider both spherically symmetric and planar converging flows, and run models with inflow Mach number from 1.1-9 to investigate the relation between core properties and the bulk velocity dispersion of the mother cloud. Four stages of protostar formation are identified: core building, core collapse, envelope infall, and late accretion. The core building stage takes 10 times as long as core collapse, which lasts a few 105 yr, consistent with observed prestellar core lifetimes. We find that the density profiles of cores during collapse can be fitted by Bonnor-Ebert sphere profiles, and that the density and velocity profiles approach the Larson-Penston solution at the core collapse instant. Core shapes change from oblate to prolate as they evolve. Cores with masses varying by three orders of magnitude ~ 0.05 - 50 solar mass are identified in our high Mach number simulations, and a much smaller mass range for models having low Mach number. The median core mass versus Mach number lies between the minimum mass that can collapse in late times Ma-1 and the most evolved core mass Ma-1/2. We implement sink particles to the grid code Athena to track the collapse of other dense regions of a large scale simulation after the most evolved core collapses, We demonstrate use of our code for applications with a simulation of planar converging supersonic turbulent flows, in which multiple cores form and collapse to create sinks; these sinks continue to interact and accrete from their surroundings over several Myr.